1. Field of the Invention
The present invention relates generally to a system for calculating current consumption characteristics of cells, such systems being used to design the layout of semiconductor integrated circuits. More particularly, this invention relates to a system for calculating current consumption characteristics of cells that takes into consideration the status change of an internal memory cell.
2. Description of the Related Art
There are apparatuses that compute power consumption based on logic simulations of semiconductor circuits in order to ensure low power consumption. As is well known, low power consumption has become necessary as integration and operation speeds of semiconductor circuits have become higher and faster. These conventional apparatuses are used to compute operational power consumption of a plurality of cells constituting a semiconductor circuit based on previously prepared current consumption characteristics of each cell in the development stage of the semiconductor circuit. Generally, the current consumption characteristics of each cell are provided by a cell characteristic calculating unit. The cell characteristic calculating unit computes the current consumption characteristics of each cell in accordance with a plurality of input terminal conditions. These input terminal conditions indicate changes in an input signal in a path which has an input terminal for receiving an input signal and an output terminal for outputting an output signal based on the input signal. Further, the operational power consumption of each cell is computed using power which is consumed at the time of charging or discharging the load capacitance of each cell. That is, cell power consumption is computed using the general equation Q=CV, where Q is a charge, C is the load capacitance, and V is a voltage. Generally, this equation is used because the charge may be taken as the sum of the current over a predetermined time.
For a cell having an internal memory, such as a sequential circuit, an input terminal condition with respect to a status change of the internal memory, as well as an input terminal condition with respect to the input-output relation, should be taken into consideration in calculating the current consumption characteristic of each cell. For a cell having an internal memory, the output signal may not vary even when the input signal has changed. Therefore, an input terminal condition for such a case should also be considered in computing the current consumption characteristics of each cell.
The reduction both in the load capacitance of cells and the power consumption due to the recent miniaturization of semiconductor circuits now demands consideration not only of the current consumption caused by charging and discharging the load capacitance of each cell, but also on the current consumption inside each cell. It is also often necessary to consider the case where the input voltage supplied from an external unit differs from the internal voltage of a cell or the case of conflicts caused by the mutual connection of the output terminals of a plurality of cells.
Accordingly, when current consumption characteristics of the individual cells are conventionally computed, a logic simulation of the semiconductor circuit is executed. Logic simulation of a semiconductor circuit having a huge number of cells may, therefore, take a considerable amount of time to determine current consumption.